The morphology and morphodynamics of sand-gravel subaquatic dunes: the Raba River estuary, Poland

Abstract

ABSTRACT:In the outlet of the Raba River to the Vistula, the biggest river in Poland, the morphology and morphodynamics of sand and finegravel subaquatic dunes were investigated. The site is situated in highland region just about the entrance to Polish Carpathians. The dunes formed on the Raba River bed estuary are composed of sand and fine gravel (d 50 up to 11 mm). Systematic observation (within the [2000][2001][2002][2003][2004][2005] were made of geometry, sediment composition and hydraulic climate under which the dunes grew and decomposed. The investigation focuses here mostly on the geometrical parameters of these bed forms such as height, length, as well as granulometric characteristics of the sediment. Based on in-site measurements different hydraulic parameters were calculated such as shear stresses, resistant coefficient, Froude and Reynolds numbers and roughness coefficient. It was found that the relation between height (H) and length (L) of the Raba estuary dunes describes the formula: H = 0.05L0.35. Also these dunes are steeper and flatter then classical H/L index is: H/L = 0.0518L0.622. During the field campaign, when the foot access to the estuary was possible and dunes were spotted on the river bed the range of measured water velocity was from v = 0.39 m•s -1 to v = 0.81 m•s -1 with the highest velocity over the dune crest. At the same time the measured range of shear stresses within the dune field formation were from  = 0.115 Nm -2 to  = 1.59 Nm -2 . On the field investigations the CCHE2D -two-dimensional unsteady flow and sediment transport model for non-equilibrium transport of non-uniform sediment mixtures -was applied. The model was used to simulate the morphodynamic changes along the outlet of the Raba River basing on field observations of the 2005 summer flood as well as calculate hydraulics parameters. It was also used to test and confirm the range of morphodynamic changes, which take place along the research reach where the dunes are being developed. The maximum water flow velocity for and the shear stresses for the summer flood caused the hydrodynamic conditions when water velocity as well as shear stresses values reached respectively up to v = 1,93 ms -1 , and  = 19.74 Nm -2 which shows that critical conditions of bed load movement were exceeded for the Raba estuary.Morphology and morphodynamics of sand-gravel subaquatic dunes: Raba River estuary, Poland 9 GEOREVIEW, Vol. 21 (2012)

Modelling Velocity Distributions and River Bed Changes Using Computer Code SSIIM below Sills Stabilizing the Riverbed

“…The parameters of bedform height such as ∆ and λ were calculated according to the Radecki-Pawlik [36] equation. The calculations were also done for k s = 3d 90 (∆ = 0).…”

“…The following Ackers-White formula is given. The sediment concentration, c, is then given in weightppm (g•m -3 ) [36]: (4) …where F gr is mobility number (m), h is depth (m•s -1 ), U is the velocity (m•s -1 ), m, n, A, C are compute parameters, and u* shear velocity (m•s -1 ).…”

Multibeam Bathymetric Investigations of the Morphology and Associated Bedforms, Sulina Channel, Danube Delta

“…Using van Rijn's classification [20] on the Yangtze River, Chen et al [16] measured dunes with maximum length of 300 m and up to 8 m in height. Small dunes were found by [15] on the Rhine River (L = 10-96 m and H = 0.3-1.4 m) and [29] on the Raba River (Poland) (L = 0.04-1.46 m and H= 0.04-0.12 m). Extremely large dunes with heights of the order of 7 m and lengths of the order of 500 m have been observed in the Rio Parana River (Argentina) at water depths of about 25 m, velocities of about 2 m/s, and bed material sizes of about 0.3 mm [20].…”